Where we live we have great view over the Firth of Forth - which is a reasonably busy waterway.
The find the full LPG tankers making their way out to the open sea fascinating as they always have a gaggle of protective tugs around them - including one going backwards behind the tanker with a cable to the stern of tanker - presumably to stop it in a hurry!
NB Yes I do wonder about the BLEVE risk - but the centre of the shipping channel is at least 2km away which you'd hope would be OK... ;-)
> including one going backwards behind the tanker with a cable to the stern of tanker - presumably to stop it in a hurry!
To act as a brake, which allows the tanker to run higher RPMs pushing more water over the rudder to increase steering authority, and also to directly steer by pulling the stern around in some cases.
> NB Yes I do wonder about the BLEVE risk - but the centre of the shipping channel is at least 2km away which you'd hope would be OK... ;-)
After the 2011 Tōhoku earthquake and tsunami, there were five BLEVE incidents with LPG tankers. Ships with up to 5000 m^3 of LPG caused fireballs of up to 500 m in diameter (1).
The largest LPG tankers into Grangemouth carry ~40,000 m^3, so if you see one on fire, I wouldn't hang around.
> I had thought that the tug was actually pulling backwards while the ship was moving forwards.
They might be doing that at very low speed, but in general an aft tow is very dangerous for a tug as it can be pulled under very easily, especially if it goes abeam/side-on to the direction of travel. Also in shallow water like some ports and estuaries, the force from the tug's props/thrusters can push against the vessel under tow cancelling out much of the force from the tow (one of the many reasons that tugs prefer to push than pull).
When a large vessel performs an emergency brake, it does so by pushing the rudder hard to one direction to use the additional drag from the hull during turning followed by going hard over the other direction to return to the original heading (it's rare a ship will have to perform an emergency stop anywhere where it has room to continue the turn - otherwise it would just continue on the new heading). This trades speed for lateral displacement and is extremely effective (although very hard on the steering gear).
However, in a congested TSS/shipping lane or river/estuary, that lateral displacement may not be available. In that case, you counter intuitively will see a tug that has an aft tow line assisting the vessel to brake by pulling 90 degrees to the side of the direction of travel/braking. This is to allow the ship to rotate in the water to push more of it's hull into the direction of travel which increases drag. Normally the resulting force would also turn the ship (ie, change the Course Over Ground, not just the heading/direction of the bow), but the tug can minimise this by pulling the stern in the opposite direction. This allows the ship slow very quickly with little lateral movement.
I'm not really on the commercial side. I'm a recreation sailor too and a marine SAR medic.
I'd say I learnt most through experience, but between IRPCS (1) and your local's port's SOPS (which are usually available online), there's a lot available. Practical ship hydrodynamics is another story, unfortunately. All the resources are very expensive.
The find the full LPG tankers making their way out to the open sea fascinating as they always have a gaggle of protective tugs around them - including one going backwards behind the tanker with a cable to the stern of tanker - presumably to stop it in a hurry!
NB Yes I do wonder about the BLEVE risk - but the centre of the shipping channel is at least 2km away which you'd hope would be OK... ;-)